[agracier]

In map_h, select the contiguous current river height rgb value and bucket fill with the rgb value of the surrounding landscape? --viking

That works OK if over the length of the river course, terrain height doesn't alter significantly. But if you start a river in a mountainous area and then let it flow to the lowlands, I don't think you can avoid canyons in the higher parts, since I don't think you can really manually adjust the terrain height all that accurately in a smooth, flowing manner.